1. Field of the Invention
The present invention relates to gate drive circuits adapted to improve the switching characteristics of devices and more particularly adapted to improve the switching characteristics of power switching devices.
2. Background Art
A bridge circuit such as an inverter generally includes a plurality of power semiconductor devices. Examples of power semiconductor devices which are widely used in bridge circuits include voltage driven power semiconductor devices such as IGBTs (Insulated Gate Bipolar Transistors) and MOSFETs. A circuit for controlling the gate voltage of a voltage driven power semiconductor device is commonly referred to as a “gate drive circuit.” That is, a gate drive circuit is used to cause a power semiconductor device to perform a switching operation.
JP-A-2001-37207 discloses an exemplary gate drive circuit for driving a power semiconductor device, which includes a turn-on side circuit for turning on the gate of the power semiconductor device, a turn-off side circuit for turning off the gate, and delay circuits. This gate drive circuit turns on the power semiconductor device in the following manner. First, a turn-on voltage is applied to a resistance connected to the gate of the power semiconductor device, so that the gate voltage gradually increases. Then, after a delay caused by a delay circuit, the same turn-on voltage is directly applied to the gate of the power semiconductor device, so that the gate voltage further increases, with the result that the device transitions to a steady on state. On the other hand, the power semiconductor device is turned off in the following manner. A turn-off voltage is applied to another resistance connected to the gate of the power semiconductor device, so that the gate voltage gradually decreases. Then, after a delay caused by another delay circuit, the same turn-off voltage is directly applied to the gate of the power semiconductor device, so that the gate voltage further decreases, with the result that the device transitions to an off state.
Thus, when turning on the power semiconductor device, the gate drive circuit of the above patent publication increases the gate voltage of the device in two steps, the second step being performed a predetermined time after the first step. Further, likewise, when turning off the power semiconductor device, the gate drive circuit decreases the gate voltage of the device in two steps, the second step being performed a predetermined time after the first step. Such gradual (or stepwise) increase and decrease in the gate voltage result in a reduced radiated noise level. Further, the switching loss is also reduced since the turn-on and turn-off times are reduced. Other prior art includes JP-A Nos. 2000-232347, 2003-189593, 2000-253646, 2003-319638, and 4-29558 (1992).
The gate drive circuit disclosed in the above JP-A-2001-37207 drives a power semiconductor device in such a way that the device exhibits a reduced radiated noise level and reduced switching loss. However, this gate drive circuit is disadvantageous in that if the arm in which the power semiconductor device is connected suffers a short circuit, a large current flows through the device since its gate voltage is relatively high, which may lead to degradation of the device. To avoid this, the gate voltage of the power semiconductor device may be reduced to increase its resistance to short circuiting. However, this results in an increase in the steady state loss in the device. Thus, with the gate drive circuit of the above patent publication, a power semiconductor device cannot exhibit sufficient resistance to short circuiting. (Decreasing its gate voltage to enhance the resistance increases the steady state loss in the device.)